A conventional computed tomography (CT) scanner includes a rotating gantry rotatably supported by a stationary gantry. The rotating gantry is configured to rotate about an examination region and carries an x-ray tube that rotates therewith. The x-ray tube is configured to emit radiation that traverses the examination region and a portion of an object or subject in the examination region. A subject support is used to position the object or subject at a static position for an axial scan or translate the object or subject through the examination region for a spiral/helical scan. A detector array is located across the examination region opposite the x-ray tube, detects radiation traversing the examination region and the object or subject, and generates projection data indicative thereof. A reconstructor reconstructs the projection data and generates volumetric of image data indicative thereof, which can be processed to generate one or more images.
Typically, before scanning the object or subject, one or more pre-scans (e.g., a scout/pilot image(s)) are performed, and the resulting data is used to facilitate creating an examination plan for the object or subject (e.g., determining the geometry of the scanned area), optimizing dose during scanning, correcting data (e.g., attenuation correction), facilitating post-processing the volumetric image data generated by a scan, etc.) For such a pre-scan, the rotating gantry and hence the x-ray tube is rotated to, if not already at, and held or maintained at a static angular position. Then, the subject support translates the object or subject through the examination region while the x-ray tube emits radiation and the detector detects radiation. The reconstructor reconstructs a two-dimensional (2D) image from the acquired data. Multiple such pre-scans have been performed for the same object or subject respectively with the x-ray tube at different angular positions.
The above-noted approach requires accelerating the rotating gantry (if the rotating gantry is not already moving) to move the x-ray tube to the pre-scan position, decelerating and stopping the rotating frame when the x-ray tube is at the requisite position, performing the pre-scan, and repeating the above steps one or more times for one or more additional pre-scans, and then accelerating the rotating frame for the scan. Unfortunately, decelerating the rotating frame for each pre-scan and accelerating the rotating frame after each pre-scan consumes time that could be spent with the patient, scanning one or more other patients, and/or otherwise used. Furthermore, when performing more than one pre-scan, the patient is moved through the examination region multiple times, one for each pre-scan, and irradiated with ionizing radiation multiple times, one for each pre-scan, and ionizing radiation may cause damage to the tissue in the irradiated region of the patient.